1. First, bring a large pot of salted water to a boil and cook the rigatoni until al dente, then drain and toss with a drizzle of oil to prevent sticking.

2. Next, heat the butter in a large skillet over medium heat and sauté the onion until translucent, about 3 to 4 minutes.

3. Then add the minced garlic and cook for 30 seconds until fragrant.

4. After that, sprinkle the flour over the onions and garlic, whisking constantly for about one minute to form a roux.

5. Slowly pour in the warm chicken broth while whisking to avoid lumps and cook until the mixture begins to thicken.

6. Stir in the cream, salt, pepper, and thyme, and let the sauce simmer for 3 to 4 minutes until it coats the back of a spoon.

7. Add the mixed vegetables and shredded chicken, stirring to combine and heat through.

8. Then fold in the cooked rigatoni, stirring until every noodle is coated with the creamy sauce.

9. Finally, let the skillet simmer for 2 to 3 more minutes to thicken, then sprinkle with parsley and Parmesan before serving.